The present invention relates to a method and circuit for monitoring the torsional vibrations of the shaft of an exciter of a turbine-generator.
In order to ensure the reliability of a turbine-generator, it is necessary to accurately determine its torsional natural frequencies in the range of approximately 5 to 130 Hz. For many turbine-generators, the torsional frequency response of the rotating exciter is considered to be a critical torsional design factor and the torsional motion of the exciter shaft must be measured during such a monitoring procedure.
It is already known in the art to measure exciter torsional motion by providing a special toothed wheel which rotates with the exciter shaft and a magnetic pickup which produces a signal having a frequency proportional to the instantaneous rate of rotation, or speed, of the toothed wheel. If a torsional vibration is present, the instantaneous shaft speed varies, thereby modulating the frequency of the magnetic pickup signal. An analog signal representative of the torsional motion is derived by frequency demodulation of the magnetic pickup signal.
While this technique produces a useful result, toothed wheels and magnetic pickups are costly and their installation often requires additional system down-time.